The present invention relates to methods and apparatus for making optical devices including microlens arrays. A microlens has an aperture or radius of less than a few millimeters. The invention is especially suitable for making microlens arrays for optical switches. Generally, the invention may be used to make optical devices for various optical coupling applications, including light conditioning and focusing to enhance optical displays, such as liquid crystal displays.
A principal application for the invention is to provide lenses for optical coupling devices. Such a device is shown in FIG. 1 for conditioning and especially collimating optical pulses traveling along an optical fiber. The end of the fiber provides a diverging beam and may be secured to a substrate on which of the lens, which collimates the beam, is disposed. The diverging beam when outputted as a collimated beam enhances the efficient propagation of the optical pulses in air, minimizing insertion loss. A prior art optical switch is shown in FIG. 2. Small lenses are needed in order to handle hundreds or thousands of input and output fibers in an optical switch arrays. These lenses allow the optical switch to be packages in a manner compatible with tiltable mirrors which route the optical pulses between selected input and output fibers of the switch. Each input fiber requires a small lens to collimate the beam, such that it may be directed and switched by the mirrors. The output lenses are in another microlens array which focuses the switched pulses to a designated output fiber. It is necessary that these microlens arrays be made to high precision, even though very small in size. Further information respecting optical switches, the routing mirrors and packaging is available in the published patents and publications for example, International Application No. WO 00/20899 published under the Patent Cooperation Treaty on Apr. 13, 2000.
Small or micro-miniature optical devices (lenses and even reflectors which are suitable for use in optical switches) are difficult to make with requisite geometrical and optical precision. Such devices and the various methods which have heretofore been proposed for their manufacture are summarized in the text entitled xe2x80x9cMicro-optics: Elements, Systems and Applicationsxe2x80x9d, published by Taylor and Francis in 1997. It has been proposed to micro-machine or electroform molds by means of which the microlens arrays may be cast, embossed or stamped. The term mold is used generically herein to indicate any tool for replication of an optical device such as a microlens array.
Further information as to such molds and their manufacture are contained in the technical literature, including by way of example the following: Hoopman et al., U.S. Pat. No. 5,300,263, issued Apr. 5, 1994; Hoopman et al., U.S. Pat. No. 5,519,539, issued May 21, 1996; Aoyama et al., U.S. Pat. No. 5,581,379, issued Dec. 3, 1996; Roffman et al., U.S. Pat. No. 5,861,114, issued Jan. 19, 1999; Schmutz, U.S. Pat. No. 5,629,765, issued May 13, 1997; Hamanaka, U.S. Pat. No. 6,069,740, issued May 30, 2000; Hamanaka et al., U.S. Pat. No. 6,129,866, issued Oct. 10, 2000; and European Published Patent Application EP 1069082 A2, published Jan. 17, 2001.
Microlens arrays have been proposed to be made directly by photolithography. Examples of this technique are in Lucitte et al., U.S. Pat. No. 4,861,140, issued Aug. 29, 1989; Iwasaki et al., U.S. Pat. No. 5,298,366, issued Mar. 29, 1994; Hoopman, U.S. Pat. No. 5,439,621, issued Aug. 8, 1995; Hamada, U.S. Pat. No. 5,453,876, issued Sep. 26, 1995; Sato et al., U.S. Pat. No. 5,801,884, issued Sep. 1, 1998; Ueda et al., U.S. Pat. No. 5,886,760, issued Mar. 23, 1999; Okazaki et al., U.S. Pat. No. 5,948,281, issued Sep. 7, 1999. Ion diffusion has also been proposed to make lenses in microlens arrays. For example, see Nakama et al., U.S. Pat. No. 5,867,321, issued Feb. 2, 1999 and U.S. Pat. No. 5,982,552, issued Nov. 9, 1999.
Photolithography, with optical and electron beams, has also been proposed to make molds by means of which microlens arrays can be produced. See Aoyama et al., U.S. Pat. No. 5,148,322, issued Sep. 15, 1992; Suzuki et al., U.S. Pat. No. 5,555,476, issued Sep. 10, 1996; and Calderini et al., U.S. Pat. No. 5,876,642, issued Mar. 2, 1999.
It is a feature of the present invention to both simplify and improve upon such heretofore proposed methods and apparatus for making optical devices and particularly microlens arrays. The optical quality in terms of selectable and precise focal lengths, focal length uniformity from lens to lens in the array, absence of aberration and surface irregularity, smooth surface finish, positional accuracy in the array of each lens, and consistency of optical centration, are obtainable with the method and apparatus of the present invention. The lenses which may be made in accordance with the invention are of such quality as to be diffraction limited, that is to have wavefront aberrations of less than a quarter wavelength of the radiation at the center of the wavelength band which is focused by the lens. The optical devices (microlens arrays) when used in pairs as in FIG. 2, because of their uniformity, positional accuracy in the array and ease of alignment introduce acceptable insertion loss in the optical switching system, which loss may be less than in the case of arrays made with heretofore proposed methods.
Another feature of the invention is that the optical devices and components which may be fabricated in accordance with the invention may be so called anamorphic optical components, for example cylindrical and toric lens.
It is a feature of the invention to be able to control the focal length of the lenses and to provide focal length uniformity from lens to lens in the microlens array. Another feature is that the surface irregularity or aberrations of the lens, as may be shown by interferometry, is minimized. Optical quality surfaces are obtainable with the invention, for example roughness of less than 50 Angstroms. The tooling used in the invention affords positional accuracy in addition to the foregoing features and enhances optical centration; that is the optical and physical axes are essentially coaxial.
It is a further feature of the invention to machine using very hard, preferably diamond, cutting tools which define lens surfaces and can be precisely rotated and/or moved with respect to a master or mold which is being machined. Chemical edging and polishing used to make the tool provides smoothness which carries over into the smoothness and regularity of the mold and the optical devices, particularly microlens arrays, replicated with the mold.
Briefly described, the invention is used in a method for making molds from which optical devices, such as microlens arrays, may be replicated. A cutting or form tool, preferably having a diamond tip, the surface of which corresponds in shape to the shape of the surface of the optical devices, is inclined, with respect to a substrate from which the mold is machined by the tool, at a precise angle dictated by the height of the optical device (the sag or radius of the each lens) replicated using the mold. The tool is rotated on the inclined axis and the cutting surface is brought into engagement with the mold surface. The shape of the tool forms a cavity or cavities in the surface of shape corresponding to the shape of the cutting surface of the tool. The point of zero velocity (where the axis of rotation crosses the cutting surface) is maintained away from the mold surface thereby ensuring that only swarf is removed or scooped out of the mold surface, preferably continuously, as each cavity is formed, avoiding chips which may adversely effect the smoothness and regularity of each cavity. The substrate, from which the molding tool is made, is preferably disposed on an X/Y table and moved to selected, different positions where other cavities are cut to provide the mold or master from which devices may be replicated. If the devices are positive lenses they may be directly cast with the mold. The mold may also be used for stamping or embossing in plastic or glass which may be in softened state during embossing. If negative or concave lenses corresponding to the cavity profiles is desired, an intermediate master may be made by casting. This master may be coated with hard or chemically (corrosive) resistant material, for example nickel, and used to mold, emboss or stamp one or more convex lenses. It will be appreciated that a complete microlens array is formed simultaneously in each replication, and each array so replicated will be consistent and identical in shape and optical properties. This makes such arrays especially suitable for use in optical switches such as shown in FIG. 2.
If optical devices with anamorphic surfaces are required, the mold (the table on which the mold is disposed) may be moved during cutting. The cutting surface may be essentially cylindrical (tooth with a cylindrical or spherical cutting tip). Cavities with toric or ellipsoidal surfaces may be cut by programming the relative velocity of displacement of the substrate and form tool during machining.